Two problems typically occur in present small cell, e.g., femto cell, deployments. In a dense deployment of small cells, e.g., femto cells, many small cell base stations are idle most of the time. However, such cells normally transmit pilot and other information even when not serving a UE (user equipment) device. While it might be desirable to control transmission of signals based on detected and decoded signals from a UE device, it may be difficult for a small cell base station to detect and decode a signal from a UE device, since the uplink (UL) transmission timing used by the UE device is normally specific to its own intended receiver and not the small cell base station which may try to detect its presence. Thus a small cell base station trying to detect UE devices in a conventional system may need to keep at least two UL timings. One UL timing is for UE devices with which the small cell base station is in communication and a second UL timing is for UE devices, which are not in communication with the small cell base station but are in communication with a macro cell base station or other small cell base station and which the small cell base station is trying to detect.
A second problem is that of interference coordination. Deployments of small cells (femtos) operating in the same band as that of a macro cell cause interference to both UL and DL (downlink) transmissions of macro and small cells. Scheduling traffic in macro and small cells may benefit from interference coordination techniques. One solution to this problem is to leave a few subframes blank for small cell transmissions. The macro cell does not use these subframes. Time division of resources creates a tradeoff between a fraction of subframes allocated to small cells and latency of small cell traffic. In LTE, it also mandates that small cells get at least a portion (e.g., ⅛th) of the resources; which may be wasteful if the small cells do not need so many resources. Frequency division of resources can address this problem. While frequency division multiplexing (FDM) between a macro cell and small cells is possible in the DL, it is not possible in the uplink in typical deployments as the UEs, often located at different distances from a base station, use different UL transmission timings. The UL transmissions are typically not synchronous both at macro and femto.
These problems are depicted in FIGS. 1 and 2. Drawing 2100 of FIG. 1 illustrates a typical deployment of a macro base station 2102 and a femto base station 2104. In FIG. 1, user equipment device 1 (UE 1) 2106 is shown communicating with the macro base station 2102; user equipment device 2 (UE 2) 2108 is shown communicating with the femto base station 2104. The transmission range of a small cell, e.g., a femto cell, is much smaller than the transmission range of a macro cell. Hence the UL and DL timing used by a UE in communication with a small cell base station, e.g., a femto base station, are almost the same. Also, note that the small cell base station acquires its DL timing by listening to DL transmissions of the macro cell.
Drawing 2200 of FIG. 2 illustrates exemplary UL and DL timing at each node (macro base station 2102, UE device 1 2106, femto base station 2104, UE device 2 2108) of FIG. 1. Horizontal line 2202 represents time. First row 2204 is used to illustrate DL and UL timing of the macro base station 2102; second row 2206 is used to illustrate DL timing of UE 1 2106 (i.e., the timing of signals received by UE 1 2106 from macro base station 2102); third row 2208 is used to illustrate UL timing of UE 1 2106 (e.g., the timing of uplink signals transmitted by UE 1 2106). Fourth row 2210 is used to illustrate timing of a UE 2 2108 transmitted signal arriving at macro base station 2102. Fifth row 2212 is used to illustrate DL and UL timing of the femto base station 2104; sixth row 2214 is used to illustrate DL timing of UE 2 2108; seventh row 2216 is used to illustrate UL timing of UE 2 2108. Eighth row 2218 is used to illustrate timing of a UE 1 2106 transmitted signal arriving at femto base station 2104. Note the discrepancy in the different uplink times. In particular with regard to the macro base station reception of uplink signals, there is a misalignment as indicated by the timing misalignment between the blocks of row 2204 and row 2210. Also, with regard to the femto base station 2104 reception of uplink signals, there is a misalignment, as indicated by the timing misalignment between the blocks of row 2212 and row 2218.
Based on the above discussion, it should be appreciated that there is a need for new methods and apparatus which can reduce interference between femto cell and macro cell communications and/or facilitate FDM in the uplink between macro cells and femto cells.